In a typical electrophotographic process a portion of a photoconductive member known as a photoreceptor is charged by a corona device to a substantially uniform potential. The charged portion is then exposed to a light pattern of an original image to selectively discharge the photoreceptor in accordance with the light pattern. The resulting pattern of charged and discharged areas form a charge pattern known as a latent image. That latent image is developed by contacting it with toner, the toner bring attracted to the image areas and held thereon by the electrostatic charge on the photoreceptor. Thus, a toner image is produced in conformity with a light pattern. The toner image is then transferred and fixed to a copy media to form a permanent record of the image. After development, any toner left on the photoreceptor is cleaned from its surface.
The foregoing discussion generally describes a typical black and white printing process. The approach utilized for multicolor electrophotographic printing is substantially the same. However, instead of forming a single latent image on the photoreceptor multiple latent images corresponding to different color separations are recorded on the photoreceptor. Each single color latent image is then developed with a toner complimentary thereto. The process is repeated for each of the images. Thereafter, the composite color image is transferred and fixed to a substrate to form a multi-layered toner image.
There are several disadvantages associated with using a photoreceptor in the electrophotographic printing process. First, once the photoreceptor surface is charged, the surface potential can only be decreased from its initial potential by discharge, making feedback correction for gray scaling and color balance difficult. Secondly, the photoreceptor cannot be used to produce a time-varying latent image, which may be useful in eliminating time dependent changes in the charge. Such time dependent changes cause higher spatial frequency components of an image to develop faster than lower frequency components. An ability to produce a time-varying latent image be useful for dry powder development to break down clusters of toner particles by spatially shifting the latent image periodically. Thirdly, the fibers on a cleaning brush that removes excess toner from the photoreceptor can easily scratch the photoreceptor surface so as to impair the copy quality of the developed image. When this happens, the operating lifetime of the photoreceptor is reduced.
Therefore, it is desirable to have a technique for performing electrophotographic printing without exposing a charged photoreceptor. Even more beneficial would be an electrophotographic printing technique that readily enables the control of the charge of a latent image.
The following disclosures may relate to various aspects of the present invention.
U.S. Pat. No. 4,588,997 PA1 Patentee: Tuan et al. PA1 Issue Date: May 13, 1986 PA1 U.S. Pat. No. 4,998,146 PA1 Patentee: Hack PA1 Issue Date: Mar. 5, 1991
The disclosures of the above-identified patents may be briefly summarized as follows:
U.S. Pat. No. 4,588,997 to Tuan et al. discloses an electrographic writing head that places continuous marks on a recording medium in response to a high voltage applied to selected writing styluses. The writing head includes a substrate upon which the stylus electrodes, multiplexed driver circuitry and active devices are integrally fabricated by thin film deposition techniques. For each stylus, there is provided a high voltage thin film transistor and a latching circuit for holding the state of the high voltage transistor for substantially an entire line writing time.
U.S. Pat. No. 4,998,146 to Hack discloses a high voltage thin film transistor having a charge transport layer. Source and drain electrodes are laterally spaced from one another and each is in a low electrical resistance contact with the charge transport layer. A gate electrode spaced normally from the source and drain electrodes extends laterally with one edge in the vicinity of the source electrode and an opposite edge located between the source and drain electrodes. A gate dielectric layer separates the gate electrode from the source and drain electrodes and the charge transport layer, in the normal direction wherein the gate electrode and the source and the drain electrodes are located on the same side of the charge transport layer.